1. Field of the Invention
The present invention relates to an electrical cable for a transmission line in either round or flat form and, in particular, to an electrical cable having a plurality of conductors therein wherein each conductor or set of conductors is physically separated and electromagnetically isolated along their entire axial length by a corrugated septum.
2. Description of the Prior Art
Round cables are presently used for relatively high speed data transmission between various system components in data processing networks. Such cables utilize twisted pairs of conductors to achieve the necessary electrical characteristics, particularly characteristic impedance and cross-talk control.
One such cable arrangement is that sold by Hewlett-Packard as the HPIB cable. This cable includes concentrically spaced inner and outer conducting members disposed about a central, axially extending core. The inner member is typically a metallized film sheath while the outer member is a metallized film sheath surrounded by a metallic braid. A first layer of twisted pairs of conductors is disposed in the annular space defined between the core and the inner surface of the inner conducting member while a second layer of twisted pairs of conductors is disposed in the annular space between the outer surface of the inner member and the inner surface of the outer conducting member. The conductors in the inner layer are used as data transmission lines while the conductors in the outer layer serve as control lines. One conductor in each twisted pair carries the appropriate data or control signal while the other of the conductors in that pair serves as the signal return for that signal. In typical usage the inner conducting member is electrically grounded and acts to isolate the data pairs from the control pairs.
A round cable assembly as described above is bulky and generally expensive to manufacture due to its complexity. Twisted conductor pairs result in an overall diameter of the twisted pair cable that is significantly larger than that of standard cables. Such a twisted pair cable can range from twenty to fifty percent larger than a standard cable depending upon conductor size and the number of conductors. These factors also result in a relatively stiffer cable construction which must be carefully fabricated in order to prevent failure due to cable flexing. Twisted pair cables often do not exhibit a uniform cross-section and can thus present problems when using automatic stripping apparatus. Furthermore, providing the appropriate terminations at each end of each cable is a relatively labor intensive endeavor since before the ends of the conductors can be terminated in a suitable connector the conductors comprising each twisted pair must be untwisted.
Despite their problems twisted pair cables are utilized because they provide electrical characteristics that are closely comparable to the electrical characteristics of coaxial cable. Of course, the cost of coaxial cable prevents its widespread use in the environment here discussed.
The cable disclosed and claimed in U.S. patent application Ser. No. 06/769,725, filed Aug. 27, 1985, a continuation-in-part Ser. No. 670,948, filed Nov. 13, 1984 both now abandoned, assigned to the present assignee provides a cable assembly using ordinary individual jacketed conductors arranged in a form that is less expensive to manufacture, less bulky and more flexible when manufactured and yet provides substantially equivalent or better electrical characteristics than are available in a cable using twisted pairs. Moreover, the relatively less expensive material cost associated with individual jacketed conductor as compared to twisted pairs leads one to form a cable from such conductors.
This cable, also known as the HPIB-II cable, uses insulated jacketed conductors arranged in an annular array in the annular space defined between an inner and an outer metallic sheath. Alternate ones of the insulated jacketed conductors in the array are designated as signal carrying conductors. The remaining conductors are electrically connected to the metallic sheaths. When the sheaths and the conductors associated with the sheaths are connected to a predetermined ground potential a cable is defined which permits each signal carrying conductor to be electrically isolated along its entire axial length. However, the grounding of alternate ones of the individual conductors eliminates their use as signal carrying conductors, thus limiting the density of the cable.
The above-mentioned application also discloses and claims a cable which overcomes this limitation by having the remaining conductors used as signal return lines. The metallic sheaths assist in partially shielding the signal carrying conductors, but a sacrifice of some electrical performance over the totally isolated case occurs.
In view of the foregoing it is believed advantageous to provide a cable structure that utilizes ordinary insulated jacketed conductors, makes maximum use of such conductors for signal carrying purposes, and yet electromagnetically isolates each signal carrying conductor along its entire axial length. In addition, it is believed advantageous to use ordinary jacketed conductors in both round and flat cable forms which maintains total electromagnetic isolation of the conductors along their entire axial length, thus approximating closely the electrical performance of a coaxial cable.